Just Purchased Virgin 340 Short - What to do with Pistons

Small Block Mopar Engine

  1. cudaracer

    cudaracer Well-Known Member

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    The 71 340 block is in great condition, and the rotating assembly spins easy. My plan is to obviously strip it down and check everything carefully, but I am not a fan of those stock 10.5:1 heavy pistons. Assuming they are in good shape, should I not stuff it with a modern set of KB's and rings?
     
  2. Oldmanmopar

    Oldmanmopar Going left turning right FABO Gold Member

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    Here's a thought , Buy a stroker kit make it a 416. Sell the stock rotating assembly .
     
  3. cudaracer

    cudaracer Well-Known Member

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    416.....pffft, If I am to stroke it, I would make it a 426ci from Hughes.

    But I purchased the short for $1K, and I am trying to keep my price low.
     
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    • Mattax

      Mattax Just the facts, ma'am FABO Gold Member

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      Then why not use them?
      CC your heads and figure what compression it will really have.
      Then go with a cam that will like that compression.
      That's low buck (cam and lifters aren't expensive compared to psitons and balancing etc) and effective use of the set up.
       
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      • yellow rose

        yellow rose Overnight Sensation FABO Gold Member

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        The KB piston doesn’t have any better ring pack than what you have.
         
      • RustyRatRod

        RustyRatRod Lemmie see your b00bs. FABO Gold Member

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        You've been reading too many magazines and listenin to too many forum gurus. Those "heavy" pistons have been around for decades. They aren't the plague. They will run fine.
         
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        • j par

          j par Well-hung Member

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          I couldn't figure out if that comment was for the OP or YR.. LOL...
           
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          • jcmeyer5

            jcmeyer5 Token Ford Guy FABO Gold Member

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            These 340's ran wicked from the factory, even with the mild cam. I agree with the others... assuming everything checks out, toss a cam in it and run it.
             
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            • Krooser

              Krooser Reform School Graduate

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              Run it....don't let other people spend your money.
               
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              • cudaracer

                cudaracer Well-Known Member

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                Great, I was hoping there would be a general consensus that stock pistons are fine, I was just checking to see if I was missing out a bunch on modern new pistons, which I know nothing about.

                So, next question, I have 65cc heads. It will take me some research to see where I would be with CR......does anybody know have the calculations handy, to report back where my CR would land? And assuming it is close to 10.5:1, how is that OK, when everything I have read, suggests I want something close to 9.5:1 with iron heads? Would I not need premium fuel or higher?
                 
              • MOPAROFFICIAL

                MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                The KB Pistons are lighter and that's the upgrade part of it ...but then you'll have to rebalance the assembly, though...you'll be removing weight and not adding, so it's on the cheaper side.
                I think if I were you... I wouldn't change the Pistons unless I was going to over bore it.. like if I decided to put new bearings and everything in it... and the bores are out round ,for instance...cause then its a waste of parts.
                 
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                • Mattax

                  Mattax Just the facts, ma'am FABO Gold Member

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                  Did you cc the heads?
                  if not, spend the time to do it.
                  There's some cheap ways will get you close.
                   
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                  • j par

                    j par Well-hung Member

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                    I was under the impression that 340s head above deck Pistons and you would either need an open chamber had or thick head gaskets?.. I'm just asking I've never had a 340...
                     
                  • ir3333

                    ir3333 Well-Known Member

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                    if those pistons are fine consider yourself lucky.If the heads are stock 71 or 72 cc's you'll be at a perfect 9.4 or 9.6 and with careful tuning, cam, gear and tire selection you'll have a little Rocket!
                     
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                    • yellow rose

                      yellow rose Overnight Sensation FABO Gold Member

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                      ^^^^^^This...and you need to know how far out of the deck the piston is. OE was .017-.018 depending on whose literature you read.

                      If you have 65 cc heads and if the piston is out of the hole you’ll need to figure out what the CR is. If you are willing to do the work you can be 10:1 on pump gas fairly easily.

                      If they are OE heads, it’s likely they aren’t that small, which is ok too because if you end up at 9.5:1 is there is nothing wrong with that either.


                      But you MUST measure to know. Guessing at CR has made engines that should run hard run like a wounded hog with no legs, so don’t guess on your CR.
                       
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                      • gm1236

                        gm1236 Well-Known Member

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                        The factory pistons are probably the lightest ones you’ll find. If you can check the deck height, most of stock motors are way under. THe piston should be .040- .045 out of the block. Like ir3333 said the head cc is most likely 70-74 cc stock. This knocks the compression down quite a bit. I had a ‘71 dart with ‘69 340 all stock, pistons, heads cast iron intake and a .480 lift cam. With 4.10, headers, car ran mid to low 13’s. Raced it for years.
                         
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                        • B3422w5

                          B3422w5 Well-Known Member

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                          I wouldnt. For the few extra cubes you get worse rod ratio, more piston side loading and wear.
                          4 inch is plenty
                          Unless you need a tractor motor to plant corn and soy beans. If so, all bets are off.... lol
                           
                          Last edited: May 12, 2020
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                          • cudaracer

                            cudaracer Well-Known Member

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                            CC's are 65, and the piston are in the bore by 0.07"

                            Can anyone calculate my CR, assuming deck height is typical
                             
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                            • Hot Metal

                              Hot Metal '69 440 Cuda

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                              Last edited: May 13, 2020
                            • 4spdragtop

                              4spdragtop CONGRATS NORTH AMERICA! FABO Gold Member

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                              No offense but if you are assembling it yourself, then you should be able to figure out compression ratio. It's a math equation based on measurements of your motor and gaskets. I'm in same situation as you. You'll get satisfaction doing it yourself.
                              Google Wallace compression calculator.
                              Give it a go and post your results.
                              Someone here will double chk results.
                              Hope I didnt sound like a dick.
                               
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                              • Wyrmrider

                                Wyrmrider FABO Gold Member FABO Gold Member

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                                how long a rod can you use using chevy rods?
                                now figure your dynamic compresson using several sceaniros
                                 
                              • Nat

                                Nat Well-Known Member

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                                nothing wrong with the stock pistons they work well , i have 2 1970 340's one totaly stock one with iron heads with a little work on them and mahle forged pistons , not much of a difference between the 2 340's except price . I'd just leave it stock until something breaks , if ever
                                 
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                                • 340doc

                                  340doc Well-Known Member

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                                  Just Fyi, check the ringlands on those stock pistons real good. The high mile or hard run ones tend to bevel the square edge on the outside of the ring groove, opening it up, making the ring looser in the groove. It once took me two sets of stock pistons to make one good set. Best of luck.
                                   
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                                  • pittsburghracer

                                    pittsburghracer FABO Gold Member FABO Gold Member

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                                    One of the funniest builds I ever did was with stock 340 pistons. I sold my buddy my 1972 Duster and he needed an engine. I told him I would build him one for 5.00 labor. He laughed and said 5.00 and I said ya go get 5.00 worth of jawbreakers and as a group we ate them while assembling his engine. Stock crank, rods, pistons, 484 purple shaft cam, reworked and gently polished set of 2.02 valve heads I sold him, portosonic intake, 850 Holley, and headers. He was a dirt track guy so this was his first drag car. He ran 12.40’s the first weekend out. We got it in the 12.20’s the next time out with some tuning and he took 2nd place. The story turned bad after the car sat for a few years and he got in with a bad crowd. We nicknamed that engine the jawbreaker engine.
                                     
                                    Last edited: May 13, 2020
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                                    • Nat

                                      Nat Well-Known Member

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                                      340 would be the 434 ci stroker , I built the hughes 426 stroker with a 360 block tons of torque but for every day driving I prefer the stockish 340 cause I'm cheap . 426 eats close to twice the amount of gas.180 km drive with the 340 used less than 1/4 tank 426 uses 1/2 a tank
                                       
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